Electrolytic production of pinacols



.Sept. 24, 1946. o. c. SLOTT ERBECK 2,403,101

ELECTROLYTIC PRODUCTION OF PINACOLS Filed July 11, 1942 LOW HYJIOGE 0Y1:

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Patented Sept. 24, 1946 ration of Delaware OLS Ober c. swim-beck, Rahway, N. 1., assignor to Standard Oil Development Company, a corpo- Application July 11, 1942, Serial N0. 450,519

6 Claims. (Cl. 204-77) 1 The present invention pertains to a method of preparing vicinal glycols by electrolytic'reductlon I of 'ketones and generally to all types of carbon to carbon reductive condensation reactions.

Specifically, this invention is concerned with the reduction of acetone electrolytically to tetramethylethylene glycol.

RI a Other side reactions result in the formation of metal alkyls, especially when the reduction is effected in acid medium with lead cathodes.

An electrolytic method for effecting the.- reductive condensation reaction whereby acetone is converted to pinacol using a lead cathode and an acid catholyte is described in German Patent 113,719 (1899).

Several German patents pertaining to the'electrolytic reduction of acetone to pinacol were is-- sued in the period 1912-1920. Among these were D. R. P. 306,304 (1917), 806,523 (1918) and 324,919 1920) The principal developments made in this period appear to have been the use of mixed metal and alloy cathodes, such as 4 to 10% copper-96 to lead mixtures and 10% tin-90% lead alloy, and the use of horizontally suspended cathodes to prevent the metal alkyls formed from accumulating on and decreasing the activity ofv the electrode surface.

All of the prior processes for electrolytically converting acetone to tetramethylethylene glycol have been unsatisfactory either because pinacol current efliciency or the overall current efllciency the general formula 2 toxic lead alkyls whichdiminish the effective- 'ness of the cathode can be minimized and the reduction oi ketones to viclnai glycols effected with good current emciency and high ratio of glycols to other reduction products by utilizin as the cathode sheets of copper or other metal of low hydrogen overvoltage' coated with zirconium. In-contrast to lead, which tends to form insoluble lead alkyls during the electrolysis, which alkyls coat the cathode and rapidly diminish the eifectivenessof the cathode, the zir-,- conium coating on the cathode forms zirconyl sulfate which is soluble in the catholyte and therefore does not coat or impair the eiiiciency of the cathode.- The zirconium coating can be easily applied to the low hydrogen overvoltage metal by electrolysis.,

Deposits of metallic zirconium were obtained from an aqueous complex sulfate bath substantially in accordance with the conditions described by Brandt and Linford, Trans. Electrochem, Soc. 70, 431 (1936). The conditions used by appiican were as follows:

Anode: Platinum sheet.

Cathode: Buifed and cleaned copper sheet.

Electrolyte: Produced by converting 18.0 g. of ZrOa' to the sulfa'te, dissolving in 100 cc. of 0.5 M. NaaSOa solution and diluting to 500 cc.

Current density: 0.12-0.-2 amp/cm.

Temperature: 20 C.

Total current passed: One ampere-hour.

It is preferred to applya zirconium coating to the low hydrogen overvoitage metal base in such 0.5 g./sq. dm.

The ketones which may be treated in accordance with the present inventioncorrespond to and aryl radicals substituted by a group which is not reducible under the conditions employed, 1. e.,

halogen, carboxyl, etc. Such ketones include, for

example acetone, methyl ethyl ketone', diethyl ketone, methyl propyl ketone, ethyl propyl ketone,' acetophenone and benzophenone.

By using mixtures of different ketones, it is possible to prepare certain mixed glycols. For

example, acetone may be condensed with methyl ethyl ketone to form the glycol corresponding to the formula or methyl ethyl ketone may be condensed for example with acetophenone to form theglycol CH; CH:

gle unit cell, and

Fig. 2 is an isometric view of the cathode of the present invention with part of the coating removed.

The single unit cell shown consists of a rectangular jar or the like arranged in a cooling bath a. An anode e,of lead or the like is arranged inside an acid resistant diaphragm d which diaphragm divides the cell into an anode and cathode compartment. As shown, 2 cathodes 0 having a zirconium coating thereon are arranged in the cell.

The details of the cell are not critical to my invention and the cell may be altered in numerous ways. For example, a number of cells could be combined in a single tank and if desired the process could be carried out continuously by providing a circulating pump, a overflow means for drawing ofi thecatholyte from the cell, means for separating electrolysis products from the withdrawn catholyte and means, for making the catholyte up to initial strength for reintroduction into the cell.

Fig. 2 illustrating the electrode inaccordance with the present invention has part of the zirconium coating removed to show the base.

The use of a diaphragm is recommended since the products formed at the cathode are liable to be oxidized at theanode. The use of a diaphragm may be avoided if desired however by the use of anolyte and catholyte of unequal densities, in cells provided with horizontal electrodes, the use of a high anodic current density or by the use of anodes having a low oxygen overvoltage, i. e., nickel or by a combination of two or more of these expedients.

Aside from the nature of the cathode surface, the factor which has the greatest effect upon the course of the reaction in the cell is the concentr. ion and type of electrolyte vused. Acid media have given the best results. The ratio of ketone to water in the catholyte' should be high. However, a ratio of 4/1 appears to be approximately the upper limit economically because of the high resistivity of the solution which would greatly increase the power cost. In the production of pinacol the preferred catholyte is one containing about 4 volumes of acetone for each volume of sul- 4 furic acid of about 20% strength. The acid, however, may vary in strength etween about 15% and about 40%. The acid strength indicated is percentage by weight.

The anolyte may be a sulfuric acid solution of about 15 to 40% strength. The electrolyte is preferably maintained at temperatures between about 0 C. and about 25 C. The current density applied may vary between about 0.1 and about 4.0 amperes/sq. din the preferred current density being within the range of about 1 to about 2 amperes/sq.dm.

The following example serves to illustrate my invention but it is to be understood that my invention is not limited thereto.

Example 1 3% liters of a 4/1 acetone-20% sulfuric acid solution were placed in the cathode compartment of a cell as shown in the drawing. The anolyte used was 2540% sulfuric acid and the anode was lead. The cathodes used are copper sheets onto which zirconium was electroplated from an aqueous solution of zirconyl sulfate and sodium sulfate as described above.

Electrolysis was effected at about 10 C. at a current density of 1.0 amperes/sq.dm. and at .6-8 volts. The run was continued for 3 hours and was 12.4/ 1. There was no metal alkyl formation on the cathode.

What I claim and desire to'secure by Letters Patent is: I

1. In the process of producing glycols corresponding to the general formula R hn hH R wherein R stands for a member of the group consisting of alkyl and aryl' radicals and substituted alkyl and aryl radicals by the electrolytic reduction of an aqueous acid solution of a ketone of the formula wherein R stands for a member of the group consisting of alkyl and aryl radicals and alkyl and aryl radicals substituted by a group which is not reducible under the reaction conditions, the improvement which comprises eflecting the said reduction at a cathode consisting of copper coated with zirconium.

2 The process of producing tetramethylethylene glycol which comprises subjecting a solution of acetone in an aqueous acid solution containlns about 15 to 40 wt. per cent of sulfuric acid to eleclytic reduction at a current density of between about 0.1 and 4-amperes per sq. dm. at a temperature between about 0 C. and about 25 C. in contact with a cathode consisting of copper coated with zirconium.

3. The process of producing tetramethylethyl- 5 of about 4 parts by volume of acetone in 1 part by volume of an aqueous solution of sulfuric acid of about 20 wt. percent strength to electrolytic reduction at a current density of between about 1 and 2 amperes per sq. dm.; in a diaphragm cell at a temperature between about 0 and 425 C. in

contact with a cathode of copper coated wlthzirconium. i

4. The process as defined in claim '1 wherein the zirconium coating on the cathode weighs about 0,01 to about 0.5 g./sq. dm.

5. The process as defined in claim 2 wherein the zirconium coating on the cathode weighs about 0.01 toabout 0.5 g./sq. dm.

6. The process as defined in claim 3 wherein the zirconium coating on the cathode weighs about 0.01 to about 0.5 gJsq. dm.

' OBER C. I" 1-: RBECK. 

